1. Field of the Invention
The present invention relates to a process of producing or manufacturing a semiconductor device, specifically a process applicable to forming metal films used as conductor layers or other components of VLSIs or the like, particularly a process capable of planarizing the metal film surface by burying viaholes with metal without causing generation of voids in the viaholes even when a mass migration occurs in the metal film during annealing after deposition.
2. Description of the Related Art
A recent trend is to provide a VLSI with an improved performance including high speed, high density, and high reliability. The VLSI has conductor layers made of a metal film usually deposited by a PVD process. Improvement of the performance of a VLSI is achieved by forming conductor layers with a refined, multiple-layered, and planarized structure. A problem has arisen in that, particularly when the conductor width or the viahole size is reduced by the refinement, it becomes difficult to ensure a good coverage when forming conductor layers in viaholes.
Therefore, it has been desired to provide a process of producing a semiconductor device, that ensures a good coverage when forming conductor layers in viaholes and is also capable of burying viaholes with metal.
Conventionally, aluminum, which is most commonly used as a conductor material among metals, is deposited by sputtering. Particularly, viaholes are buried by a high temperature sputtering or a high temperature bias sputtering, in which the substrate temperature is raised to a temperature near the melting point of the metal being deposited to form a metal film.
Viaholes are also buried by another method in which a metal film deposited by a usual sputtering is temporarily melted by laser irradiation. Viaholes are alternatively buried by a method in which a selective CVD process or a blanket CVD process is combined with an etch-back process.
The above-recited conventional usual sputtering, however, has a problem that, when a viahole has a small diameter and an large depth or has a large aspect ratio (depth to diameter ratio), the amount of metal deposited on the side wall and the bottom of the viahole is far less than that deposited on the substrate portion outside the viahole. Moreover, at the open top of the viahole, the deposited metal forms an overhang to consequently block the viahole in the top. It is thus difficult to deposit metal in a viahole and a metal film formed in the viahole contains voids, i.e., it is difficult to provide good coverage when forming a metal film in a viahole.
Although the above-mentioned conventional high temperature sputtering or high temperature bias sputtering is advantageous over the usual sputtering in that an overhang is harder to form than the usual sputtering because the substrate is held at a high temperature during deposition of a metal film, it has the drawback that the high substrate temperature causes the deposited metal to easily form balls and makes it difficult to provide a continuous film. To suppress ball formation, the substrate temperature must be occasionally varied, so that the process is difficult to control and cannot be stably operated.
Although the herein first-recited process performing annealing after deposition at room temperature is advantageous in that viaholes can be buried by deforming the deposited metal by annealing, it still has the problem specific to the room temperature deposition process in that, with an increased aspect ratio of viaholes, the deposited metal forms an overhang at the viahole top and thereby makes it difficult to form a metal film in viaholes with a good coverage, with the result that, after the annealing, the viaholes have a blocked top and contain voids left therein.
The conventional selective CVD and blanket CVD processes have also a problem that, because a metal film is deposited through a reaction with a substrate surface, the deposition process is sensitive to the surface conditions of the substrate and is more expensive than the sputtering process.